1. Field of the Invention
The present invention relates generally to HSDPA communication, and in particular, to a method for transmitting common information shared by UEs (User Equipments) supporting an HSDPA service.
2. Description of the Related Art
In general, HSDPA (High-Speed Downlink Packet Access) refers to a data transmission technique for handling a high-speed downlink shared channel (HS-DSCH), i.e., a downlink data channel for supporting high-speed downlink packet data transmission, and its associated control channel in an UMTS (Universal Mobile Telecommunications System) communication system. In order to support the HSDPA, AMC (Adaptive Modulation and Coding), HARQ (Hybrid Automatic Retransmission Request), and FCS (Fast Cell Select) have been proposed.
1) AMC
AMC is a data transmission technique for adaptively determining a modulation technique and a coding technique of different data channels according to a channel condition between a Node B and a UE, thereby increasing the overall utilization efficiency of the cell. Therefore, AMC involves a plurality of modulation techniques and a plurality of coding techniques, and modulates and codes data channels by combining the modulation techniques and the coding techniques. Generally, each combination of the modulation techniques and the coding techniques is called a “MCS (Modulation and Coding Scheme)”, and there are a plurality of MCSs with level #1 to level #n according to the number of combinations of the modulation techniques and the coding techniques.
In other words, AMC adaptively determines a level of the MCS according to a channel condition between the Node B and the UE that is currently wirelessly connected to the Node B, thereby increasing the overall efficiency of the Node B.
2) HARQ
HARQ, especially n-channel SAW HARQ (n-channel Stop And Wait Hybrid Automatic Retransmission Request), will be described below. HARQ has introduced the following two plans to increase transmission efficiency of ARQ (Automatic Retransmission Request). In a first plan, HARQ exchanges a data retransmission request and a response between the UE and the Node B. In a second plan, HARQ temporarily stores defective data and combines it with corresponding retransmitted data before transmission. Further, the HSDPA has introduced n-channel SAW HARQ in order to make up for the shortcomings of the conventional SAW ARQ (Stop And Wait ARQ). In the SAW ARQ, the Node B does not transmit the next packet data until an ACK (Acknowledgement) for the previously transmitted packet data is received. Therefore, in some cases, the Node B must await the ACK, though it can presently transmit packet data.
The n-channel SAW HARQ increases channel utilization efficiency by continuously transmitting a plurality of data packets before receiving the ACK for the previous packet data. If n logical channels are established between a UE and a Node B and identified by time or channel numbers, the UE, upon receipt of packet data at a certain time point, can determine the logical channel that transmitted the packet data. Thus the ULE can rearrange packet data in the right reception order or soft-combine the packet data.
3) FCS
FCS rapidly selects a cell having a good channel condition among a plurality of cells, when a UE supporting the HSDPA enters a cell-overlapped region, or a soft handover region. To be more specific, if the UE supporting the HSDPA enters a cell-overlapped region between an old Node B and a new Node B, then the UE establishes radio links to a plurality of the cells, i.e., a plurality of Node Bs. A set of the cells, to which the radio links are established by the UE, is called an “active set.” FCS receives HSDPA packet data from only the cell maintaining the best channel condition among the cells included in the active set, thereby reducing the overall interference. Herein, a cell transmitting the HSDPA packet data for its best channel condition among the cells in the active set is called a “best cell.” The UE periodically checks the channel conditions with the cells belonging to the active set. Upon detecting a cell having a channel condition better than that of the current best cell, the UE transmits a best cell indicator to all of the cells in the active set in order to replace the current best cell with a new best cell. The best cell indicator includes an identifier of the selected new best cell. Upon receiving the best cell indicator, the cells belonging to the active set analyze the cell identifier included in the received best cell indicator to determine whether the received best cell indicator is destined for them. The selected best cell transmits packet data to the UE using a high-speed downlink shared channel (HS-DSCH).
As described above, in the HSDPA, it is necessary to exchange the following new control signals between a UE and a Node B in order to support the newly introduced AMC, HARQ, and FCS. First, in order to support AMC, a UE must provide information on a channel between the UE and a Node B to the Node B, and the Node B must inform the UE of an MCS level determined according to the channel condition using the channel information received from the UE. Second, in order to support n-channel SAW HARQ, a UE must transmit an ACK or a NACK (Negative Acknowledgement) signal to a Node B, and the Node B must transmit n-channel SAW information to the UE. The n-channel SAW information can be a channel number and a new/continue flag, or a sequence number (SN). Third, in order to support FCS, the UE must transmit to the Node B a best cell indicator indicating a best cell, i.e., a Node B providing a channel with the best channel condition. In addition, if the best cell is changed according to the channel condition, the UE must inform the Node B of its packet data reception state at that point, and the Node B should provide necessary information so that the UE can correctly select the best cell.
The MCS level information, the HARQ information including the channel number and the new/continue flag, and the FCS information must be previously transmitted before a Node B selects a UE and transmits data to the selected UE. By previously transmitting such information before transmitting the data, the Node and the UE can perform the same operation using the information. HSDPA proposes two methods of transmitting the information to the UE; a 2-step method and a 1-step method. First, the 2-step method will be described with reference to FIG. 1.
FIG. 1 illustrates a schematic channel structure to which a 2-step method of transmitting HSDPA information is applied, in a conventional HSDPA communication system. It will be assumed in FIG. 1 that there are three UEs supporting the HSDPA service, two shared control channels (SHCCHs) assigned, and three high-speed physical downlink shared channels (HS-PDSCHs) assigned. The SHCCH, a shared channel newly designed to support the HSDPA service, transmits control information to a UE scheduled to receive the HSDPA data. For example, a high-speed shared control channel (HS-SCCH) is a typical SHCCH. The HS-PDSCH, a physical channel for transmitting the HS-DSCH, is identified by a scrambling code and an OVSF (Orthogonal Variable Spreading Factor) code. When the HSDPA service is provided, one or more HS-PDSCHs can be assigned and all UEs must recognize information on the HS-PDSCH codes.
As illustrated in FIG. 1, the HSDPA data is transmitted at a period of TTI (Transmission Time Interval), and a Node B selects a UE that will receive the HSDPA data in each TTI, and informs the corresponding UE that there exist HSDPA data to receive, over a dedicated physical channel (DPCH). To inform the corresponding UE that the HSDPA data is to be transmitted, the Node B provides the corresponding information through an HSDPA indication (HI) in each TTI of a DPCH established between the UE and the Node B. The HI is transmitted over the DPCH assigned to each UE. If a specific bit of the HI has a preset value, the UE will receive the HSDPA data in the next TTI of the HI reception point. If the specific bit of the HI does not have the preset value, the UE does not receive the HSDPA data in the next TTI of the HI reception point.
Therefore, the UE receives the HI in each TTI of the DPCH and determines whether there is HSDPA data to receive. That is, if there is data to transmit to the UE, the Node B sets a specific bit of the HI to a set value indicating that the HSDPA data exists, before transmission. The UE then receives the HI and recognizes that the HSDPA data will be received in the next TTI of the HI reception point. Actually, the HI is received in each TTI on the DPCHs assigned to the respective UEs, i.e., UE1, UE2 and UE3. However, for the sake of convenience, only the HI indicating existence of the HSDPA data is illustrated in FIG. 1.
Upon detecting the expected receipt of the HSDPA data, the UE searches SHCCH transmitted in the next TTI. If there are a plurality of the SHCCHs assigned, UE ID can be included in the SHCCH in order to identify the UE. Therefore, the UE receives information on the SHCCH transmitted with the same ULE ID as its own UE ID among the SHCCHs. Meanwhile, if the HI, as stated above, transmits only the information indicating existence of the HSDPA data to be received at the UE, the ULE ID for identifying the UE must be inserted in the SHCCH before being transmitted. However, if the HI includes information indicating existence of the HSDPA data to be received at the UE and information on the SHCCH that the UE must receive, the UE ID for identifying the UEs may not be included in the SCCH. Following receipt of the SHCCH information, the UE receives the HSDPA data in the corresponding TTI. Here, the information on the SHCCH includes UE ID, MCS level, and HARQ information, and it is assumed in FIG. 1 that the SHCCH information and the HSDPA data are transmitted in the same TTI. However, in order to minimize the number of reception buffers, it is preferable to transmit the SHCCH information ahead of HS-PDSCH. In this case, the UE can simultaneously receive the HI and the SHCCH.
FIG. 2 illustrates a schematic channel structure to which a 1-step method of transmitting HSDPA information is applied, in a conventional HSDPA communication system. It will also be assumed in FIG. 2 that there are three UEs supporting the HSDPA service, two SHCCHs assigned, and three HS-PDSCHs assigned.
As illustrated in FIG. 2, the HI information is not transmitted over the DPCHs received by the UEs in the 1-step method. Therefore, all UEs should continuously receive the SHCCH signal, and the SHCCH should include UE ID in order to designate a target UE to which the corresponding HSDPA information should be transmitted. The UE receives the SHCCH signal, and analyzes the SHCCH signal if the received SHCCH signal includes its own UE ID. Based on the analyzed results, the UE receives data on a corresponding HS-PDSCH.
A description will now be made of Information Group #1 including the information that must be transmitted from the Node B to the UE in order to support the HSDPA service.
Information Group #1
1) HI: this indicates whether there is HSDPA data that the UE should receive.
2) MCS level information: this indicates an AMC scheme to be used in modulating the HS-DSCH.
3) HS-DSCH channelization code information: this indicates information related to a channelization code used for a specific UE in the HS-DSCH.
4) HARQ processor number: this indicates a channel transmitting specific packet data among logical channels for the HARQ, when n-channel SAW HARQ is used. The reason for providing the HARQ processor number information is as follows. When n-channel SAW HARQ is used, as many HARQ processors as the number ‘n’ of the channels must be provided. Therefore, when corresponding packet data is transmitted over a certain channel and the transmitted packet data has an error, it is necessary to recognize the HARQ processor number corresponding to a transmission channel of the errored packet data. That is, among the packet data transmitted from the HARQ processor corresponding to the channel number over the corresponding channel, the defective packet data is subject to a retransmission process.
5) HARQ packet data number: this is used to inform the UE of a downlink packet data number so that when a best cell is changed in the FCS, the UE can inform a newly selected best cell of a transmission state of the HSDPA data. As the packet data has its own sequence number, it can be identified by the sequence number.
6) UE ID (Identification): when it is transmitted through a part of the above information and there is at least one SHCCH, the UE ID indicates whether the SHCCH is control information for the corresponding UE.
In addition to Information Group #1, the information that must be transmitted to support the HSDPA service may include an uplink transmission power offset value. When the UE provides information of the selected best cell to neighboring Node Bs, i.e., Node Bs in the active set other than the current best cell, the UE must increase uplink transmission power by an offset so that the neighboring Node Bs can correctly receive the best cell information transmitted by the UE. In this case, the uplink transmission power offset value is provided from the Node B to the UE. In addition, the information transmitted to support the HSDPA service may include information indicating retransmission and information indicating order of the retransmission, in an HARQ operation.
The above information, uniquely given to each UE, must be transmitted to the UE ahead of an HS-DSCH signal. Therefore, there is a demand for a method of transmitting the above information ahead of the HS-DSCH signal, i.e., actual packet data. Further, the current asynchronous mobile communication system does not support a method of broadcasting information shared by all UEs in service such as the information used in the HSDPA. Thus, it is necessary to apply the HSDPA information generated by the Node B or an RNC (Radio Network Controller) to all the UEs, when the HSDPA is used.